Emergency department (ED) crowding has strained the acute care system to its breaking point. Two of the most visible symptoms of ED crowding, ambulance diversion and 'left before being seen'visit rates, have dramatically increased over time and are now routine in many urban centers. ED crowding causes delays in evaluation and treatment, which in turn may contribute to adverse outcomes. Consequently, the American College of Emergency Physicians has identified ED crowding as one of the top problems threatening the safety and quality of emergency care. However, policymakers and health system leaders have failed to address emergency department crowding as a top public health priority. The primary reason for inaction is the paucity of data on community disparities in ED crowding and the impact of ED crowding on patient outcomes. Without such evidence, policymakers are unlikely to commit substantial resources to address root causes of ED crowding. In order to assess patterns and outcomes of ED crowding, we propose a retrospective cohort study of all 2007 ED visits to non-federal, California hospitals. This analysis will include 8.8 million discharges and 1.5 million hospitalizations from 342 California EDs. We will use daily ambulance diversion hours and 'left before being seen'(LBBS) visit rates as measures of ED crowding. This study has the following Specific Aims: 1. Identify community level predictors of emergency department crowding. 2. Assess the relationship between emergency department crowding and 7-day bounce-back hospitalizations after emergency department discharge. 3. Assess the relationship between emergency department crowding and inpatient outcomes, including death, length-of-stay, and costs. California accounts for nearly 10% of all ED visits in the United States and represents a diversity of patient populations and hospital systems. Therefore, the proposed study will provide generalizable insights into this health policy crisis.